| 1. |
- Abreu, L. I., et al.
(författare)
-
Reduced-order models to analyse coherent structures in turbulent pipe flow
- 2019
-
Ingår i: 11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019. - : International Symposium on Turbulence and Shear Flow Phenomena, TSFP.
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Konferensbidrag (refereegranskat)abstract
- Fully resolved direct numerical simulations, performed with a high-order spectral-element method, are used to study coherent structures in turbulent pipe flow at friction Reynolds numbers Reτ = 180 and 550 (El Khoury et al., 2013). The database was analysed using spectral proper orthogonal decomposition (SPOD) so as to identify dominant coherent structures, most of which are of streaky shape. As a reduced-order model for such structures, the linearised flow response to harmonic forcing was computed, and the analysed singular modes of the resolvent operator were analysed. For turbulent flows, this approach amounts to considering the non-linear terms in the Navier–Stokes system as an unknown forcing, treated convenienty as external. Resolvent analysis then allows an identification of the optimal forcing and most amplified flow response; the latter may be related to observed relevant structures obtained by SPOD, especially if the gain between forcing and response is much larger than what is found for suboptimal forcings or if the non-linear forcing is white noise. Results from SPOD and resolvent analysis were extracted for several combinations of frequencies, streamwise and azimuthal wavenumbers. For both Reynolds numbers, good agreement between SPOD and resolvent modes was observed for parameter combinations where the lift-up mechanism is present: optimal forcing from resolvent analysis represents streamwise vortices and the associated response are streaky structures.
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| 2. |
- Abreu, L. I., et al.
(författare)
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Wavepackets in turbulent flow over a NACA 4412 airfoil
- 2018
-
Ingår i: 31st Congress of the International Council of the Aeronautical Sciences, ICAS 2018. - : International Council of the Aeronautical Sciences. - 9783932182884
-
Konferensbidrag (refereegranskat)abstract
- Turbulent flow over a NACA 4412 airfoil with an angle of attack AoA = 5◦ was analysed using an incompressible direct numerical simulation (DNS) at chord Reynolds number of Rec = 4 · 105. Snapshots of the flow field were analysed using the method of Spectral Proper Orthogonal Decomposition (SPOD) in frequency domain, in order to extract the dominant coherent structures of the flow. Focus is given to two-dimensional disturbances, known to be most relevant for aeroacoustics. The leading SPOD modes show coherent structures forming a wavepacket, with significant amplitudes in the trailing-edge boundary layer and in the wake. To model coherent structures in the turbulent boundary layer, the optimal harmonic forcing and the associated linear response of the flow were obtained using the singular value decomposition of the linear resolvent operator. The resolvent analysis shows that the leading SPOD modes can be associated to most amplified, linearised flow responses. Furthermore, coherent structures in the wake are modelled as the Kelvin-Helmholtz mode from linear stability theory (LST).
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| 3. |
- Atzori, Marco, et al.
(författare)
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Characterization of turbulent coherent structures in square duct flow
- 2018
-
Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP).
-
Konferensbidrag (refereegranskat)abstract
- This work is aimed at a first characterization of coherent structures in turbulent square duct flows. Coherent structures are defined as connected components in the domain identified as places where a quantity of interest (such as Reynolds stress or vorticity) is larger than a prescribed non-uniform threshold. Firstly, we qualitatively discuss how a percolation analysis can be used to assess the effectiveness of the threshold function, and how it can be affected by statistical uncertainty. Secondly, various physical quantities that are expected to play an important role in the dynamics of the secondary flow of Prandtl's second kind are studied. Furthermore, a characterization of intense Reynolds-stress events in square duct flow, together with a comparison of their shape for analogous events in channel flow at the same Reynolds number, is presented.
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| 4. |
- Atzori, Marco, 1992-, et al.
(författare)
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Contribution of Reynolds-stress structures to the secondary flow in turbulent ducts
- 2019
-
Ingår i: 11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019. - : International Symposium on Turbulence and Shear Flow Phenomena, TSFP.
-
Konferensbidrag (refereegranskat)abstract
- The present work is aimed at evaluating the contribution to the secondary flow in duct flow with square and rectangular cross section from three-dimensional coherent structures, defined as intense Reynolds-stress events. The contribution to a certain mean quantity is defined as the ensemble average over the detected coherent structures, weighted with their own occupied volume fraction. Our analysis unveils that the contribution to the cross-stream components of the mean velocity is either very similar to the same contribution in channel flow, or almost negligible in respect to the contribution from the portion of the domain not occupied by coherent structures. These results suggest that the most intense events are not directly responsible for the secondary flow.
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| 5. |
- Bobke, Alexandra, et al.
(författare)
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History effects and near equilibrium in adverse-pressure-gradient turbulent boundary layers
- 2017
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 820, s. 667-692
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent boundary layers under adverse pressure gradients are studied using well-resolved large-eddy simulations (LES) with the goal of assessing the influence of the streamwise pressure-gradient development. Near-equilibrium boundary layers were characterized through the Clauser pressure-gradient parameter β. In order to fulfil the near-equilibrium conditions, the free stream velocity was prescribed such that it followed a power-law distribution. The turbulence statistics pertaining to cases with a constant value of β (extending up to approximately 40 boundary-layer thicknesses) were compared with cases with non-constant β distributions at matched values of β and friction Reynolds number Reδ∗. An additional case at matched Reynolds number based on displacement thickness Reδ∗ was also considered. It was noticed that non-constant β cases appear to approach the conditions of equivalent constant β cases after long streamwise distances (approximately 7 boundary-layer thicknesses). The relevance of the constant β cases lies in the fact that they define a 'canonical' state of the boundary layer, uniquely characterized by β and Re. The investigations on the flat plate were extended to the flow around a wing section overlapping in terms of β and Re. Comparisons with the flat-plate cases at matched values of β and Re revealed that the different development history of the turbulent boundary layer on the wing section leads to a less pronounced wake in the mean velocity as well as a weaker second peak in the Reynolds stresses. This is due to the weaker accumulated effect of the β history. Furthermore, a scaling law suggested by Kitsios et al. (Intl J. Heat Fluid Flow, vol. 61, 2016, pp. 129-136), proposing the edge velocity and the displacement thickness as scaling parameters, was tested on two constant-pressure-gradient parameter cases. The mean velocity and Reynolds-stress profiles were found to be dependent on the downstream development. The present work is the first step towards assessing history effects in adverse-pressure-gradient turbulent boundary layers and highlights the fact that the values of the Clauser pressure-gradient parameter and the Reynolds number are not sufficient to characterize the state of the boundary layer.
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| 6. |
- Bobke, Alexandra, et al.
(författare)
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Large-eddy simulations of adverse pressure gradient turbulent boundary layers
- 2016
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Ingår i: 2nd Multiflow Summer School on Turbulence. - : Institute of Physics (IOP).
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Konferensbidrag (refereegranskat)abstract
- Adverse pressure-gradient (APG) turbulent boundary layers (TBL) are studied by performing well-resolved large-eddy simulations. The pressure gradient is imposed by defining the free-stream velocity distribution with the description of a power law. Different inflow conditions, box sizes and upper boundary conditions are tested in order to determine the final set-up. The statistics of turbulent boundary layers with two different power-law coefficients and thus magnitudes of adverse pressure gradients are then compared to zero pressure-gradient (ZPG) data. The effect of the APG on TBLs is manifested in the mean flow through a much more prominent wake region and in the Reynolds stresses through the existence of an outer peak. The pre-multiplied energy budgets show, that more energy is transported from the near-wall region to farther away from the wall.
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| 7. |
- Chin, C., et al.
(författare)
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Flow topology of rare back flow events and critical points in turbulent channels and toroidal pipes
- 2018
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP).
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Konferensbidrag (refereegranskat)abstract
- A study of the back flow events and critical points in the flow through a toroidal pipe at friction Reynolds number Reτ ≈ 650 is performed and compared with the results in a turbulent channel flow at Reτ ≈ 934. The statistics and topological properties of the back flow events are analysed and discussed. Conditionally-averaged flow fields in the vicinity of the back flow event are obtained, and the results for the torus show a similar streamwise wall-shear stress topology which varies considerably for the spanwise wall-shear stress when compared to the channel flow. The comparison between the toroidal pipe and channel flows also shows fewer back flow events and critical points in the torus. This cannot be solely attributed to differences in Reynolds number, but is a clear effect of the secondary flow present in the toroidal pipe. A possible mechanism is the effect of the secondary flow present in the torus, which convects momentum from the inner to the outer bend through the core of the pipe, and back from the outer to the inner bend through the pipe walls. In the region around the critical points, the skin-friction streamlines and vorticity lines exhibit similar flow characteristics with a node and saddle pair for both flows. These results indicate that back flow events and critical points are genuine features of wall-bounded turbulence, and are not artifacts of specific boundary or inflow conditions in simulations and/or measurement uncertainties in experiments.
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| 8. |
- Dogan, Eda, et al.
(författare)
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Quantification of amplitude modulation in wall-bounded turbulence
- 2019
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Ingår i: Fluid Dynamics Research. - : IOP PUBLISHING LTD. - 0169-5983 .- 1873-7005. ; 51:1
-
Tidskriftsartikel (refereegranskat)abstract
- Many recent investigations on the scale interactions in wall-bounded turbulent flows focus on describing so-called amplitude modulation, the phenomenon that deals with the influence of large scales in the outer region on the amplitude of the small-scale fluctuations in the near-wall region. The present study revisits this phenomenon regarding two aspects, namely the method for decomposing the scales and the quantification of the modulation. First, the paper presents a summary of the literature that has dealt with either or both aspects. Second, for decomposing the scales, different spectral filters (temporal, spatial or both) and empirical mode decomposition (EMD) are evaluated and compared. The common data set is a well-resolved large-eddy simulation that offers a wide range of Reynolds numbers spanning Re-theta = 880-8200. The quantification of the amplitude modulation is discussed for the resulting scale components. Particular focus is given to evaluate the efficacy of the various filters to separate scales for the range of Reynolds numbers of interest. Different to previous studies, the different methods have been evaluated using the same data set, thereby allowing a fair comparison between the various approaches. It is observed that using a spectral filter in the spanwise direction is an effective approach to separate the small and large scales in the flow, even at comparably low Reynolds numbers, whereas filtering in time should be approached with caution in the low-to-moderate Re range. Additionally, using filters in both spanwise and time directions, which would separate both wide and long-living structures from the small and fast scales, gives a cleaner image for the small-scales although the contribution to the scales interaction from that filter implementation has been found negligible. Applying EMD to decompose the scales gives similar results to Fourier filters for the energy content of the scales and thereby for the quantification of the amplitude modulation using the decomposed scales. No direct advantage of EMD over classical Fourier filters could be seen. Potential issues regarding different decomposition methods and different definitions of the amplitude modulation are also discussed.
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| 9. |
- Edwards, Robert A., et al.
(författare)
-
Global phylogeography and ancient evolution of the widespread human gut virus crAssphage
- 2019
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Ingår i: Nature Microbiology. - : Springer Science and Business Media LLC. - 2058-5276. ; 4:10, s. 1727-1736
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Tidskriftsartikel (refereegranskat)abstract
- Microbiomes are vast communities of microorganisms and viruses that populate all natural ecosystems. Viruses have been considered to be the most variable component of microbiomes, as supported by virome surveys and examples of high genomic mosaicism. However, recent evidence suggests that the human gut virome is remarkably stable compared with that of other environments. Here, we investigate the origin, evolution and epidemiology of crAssphage, a widespread human gut virus. Through a global collaboration, we obtained DNA sequences of crAssphage from more than one-third of the world's countries and showed that the phylogeography of crAssphage is locally clustered within countries, cities and individuals. We also found fully colinear crAssphage-like genomes in both Old-World and New-World primates, suggesting that the association of crAssphage with primates may be millions of years old. Finally, by exploiting a large cohort of more than 1,000 individuals, we tested whether crAssphage is associated with bacterial taxonomic groups of the gut microbiome, diverse human health parameters and a wide range of dietary factors. We identified strong correlations with different clades of bacteria that are related to Bacteroidetes and weak associations with several diet categories, but no significant association with health or disease. We conclude that crAssphage is a benign cosmopolitan virus that may have coevolved with the human lineage and is an integral part of the normal human gut virome.
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| 10. |
- Friederici, Anke, 1994-, et al.
(författare)
-
Distributed Percolation Analysis for Turbulent Flows
- 2019
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Ingår i: 2019 IEEE 9th Symposium on Large Data Analysis and Visualization, LDAV 2019. - : Institute of Electrical and Electronics Engineers (IEEE). - 9781728126050 ; , s. 42-51
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Konferensbidrag (refereegranskat)abstract
- Percolation analysis is a valuable tool to study the statistical properties of turbulent flows. It is based on computing the percolation function for a derived scalar field, thereby quantifying the relative volume of the largest connected component in a superlevel set for a decreasing threshold. We propose a novel memory-distributed parallel algorithm to finely sample the percolation function. It is based on a parallel version of the union-find algorithm interleaved with a global synchronization step for each threshold sample. The efficiency of this algorithm stems from the fact that operations in-between threshold samples can be freely reordered, are mostly local and thus require no inter-process communication. Our algorithm is significantly faster than previous algorithms for this purpose, and is neither constrained by memory size nor number of compute nodes compared to the conceptually related algorithm for extracting augmented merge trees. This makes percolation analysis much more accessible in a large range of scenarios. We explore the scaling of our algorithm for different data sizes, number of samples and number of MPI processes. We demonstrate the utility of percolation analysis using large turbulent flow data sets.
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| 11. |
- Guastoni, L., et al.
(författare)
-
On the use of recurrent neural networks for predictions of turbulent flows
- 2019
-
Ingår i: 11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019. - : International Symposium on Turbulence and Shear Flow Phenomena, TSFP.
-
Konferensbidrag (refereegranskat)abstract
- In this paper, the prediction capabilities of recurrent neural networks are assessed in the low-order model of near-wall turbulence by Moehlis et al. (New J. Phys. 6, 56, 2004). Our results show that it is possible to obtain excellent predictions of the turbulence statistics and the dynamic behavior of the flow with properly trained long short-term memory (LSTM) networks, leading to relative errors in the mean and the fluctuations below 1%. We also observe that using a loss function based only on the instantaneous predictions of the flow may not lead to the best predictions in terms of turbulence statistics, and it is necessary to define a stopping criterion based on the computed statistics. Furthermore, more sophisticated loss functions, including not only the instantaneous predictions but also the averaged behavior of the flow, may lead to much faster neural network training.
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| 12. |
- Guemes, A., et al.
(författare)
-
Flow organization in the wake of a rib in a turbulent boundary layer with pressure gradient
- 2019
-
Ingår i: Experimental Thermal and Fluid Science. - : ELSEVIER SCIENCE INC. - 0894-1777 .- 1879-2286. ; 108, s. 115-124
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of a streamwise pressure gradient on the wake developed by wall-attached square ribs in a turbulent boundary layer is investigated experimentally. Favourable-, adverse- and zero-pressure-gradient conditions (FPG, APG and ZPG, respectively) are reproduced at matched friction Reynolds number and non-dimensional rib height. Flow-field measurements are carried out by means of Particle Image Velocimetry (PIV). Turbulence statistics are extracted at high resolution using an Ensemble Particle Tracking Velocimetry approach. Modal analysis is performed with Proper Orthogonal Decomposition (POD). We demonstrate that a non-dimensional expression of the pressure gradient and shear stress is needed to quantify the pressure-gradient effects in the wake developing past wall-attached ribs. We suggest the Clauser pressure-gradient parameter beta, commonly used in the literature for the characterization of turbulent boundary layers under the effect of a pressure gradient, as a suitable parameter. The results show that, in presence of an adverse pressure gradient, the recirculation region downstream of the rib is increased in size, thus delaying the reattachment, and that the peak of turbulence intensity and the shed eddies are shifted towards larger wall-normal distances than in the ZPG case. The observed changes with respect to the ZPG configuration appear more intense for larger magnitude of beta, which are more likely to be obtained in APG than in FPG due to the reduced skin friction and increased displacement thickness.
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| 13. |
- Hosseini, Seyed M., et al.
(författare)
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Direct numerical simulation of the flow around a wing section at moderate Reynolds number
- 2016
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Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier. - 0142-727X .- 1879-2278. ; 61, s. 117-128
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Abstract A three-dimensional direct numerical simulation has been performed to study the turbulent flow around the asymmetric NACA4412 wing section at a moderate chord Reynolds number of R e c = 400 , 000 , with an angle of attack of A o A = 5 ∘ . The mesh was optimized to properly resolve all relevant scales in the flow, and comprises around 3.2 billion grid points. The incompressible spectral-element Navier–Stokes solver Nek5000 was used to carry out the simulation. An unsteady volume force is used to trip the flow to turbulence on both sides of the wing at 10% of the chord. Full turbulence statistics are computed in addition to collection of time history data in selected regions. The Reynolds numbers on the suction side reach Reτ ≃ 373 and R e Ξ = 2 , 800 with the pressure-gradient parameter ranging from β ≈ 0.0 to β ≈ 85. Similarly, on the pressure side, the Reynolds numbers reach Reτ ≈ 346 and R e Ξ = 818 while β changes from β ≈ 0.0 to β ≈ − 0.25 . The effect of adverse pressure gradients on the mean flow is consistent with previous observations, namely a steeper incipient log law, a more prominent wake region and a lower friction. The turbulence kinetic energy profiles show a progressively larger inner peak for increasing pressure gradient, as well as the emergence and development of an outer peak with stronger APGs. The present simulation shows the potential of high-order (spectral) methods in simulating complex external flows at moderately high Reynolds numbers.
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| 14. |
- Hosseini, Seyed M., et al.
(författare)
-
Direct numerical simulation of the flow around a wing section at moderate Reynolds numbers
- 2015
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Ingår i: Proceedings - 15th European Turbulence Conference, ETC 2015. - : TU Delft.
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Konferensbidrag (refereegranskat)abstract
- A three dimensional direct numerical simulation has been performed to study the flow around the asymmetric NACA-4412 wing at a moderate chord Reynolds number (Rec = 400, 000) with an angle of attack of 5◦. The flow case under investigation poses numerous challenges for a numerical method due to the wide range of scales and complicated flow physics induced by the geometry. The mesh is optimized and well resolved to account for such varying scales in the flow. An unsteady volume force is used to trip the flow to turbulence on both sides of the wing at 10% chord. Full turbulent statistics are computed on the fly to further investigate the complicated flow features around the wing. The present simulation shows the potential of high-order methods in simulating complex external flows at moderately high Reynolds numbers.
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| 15. |
- Karnama, A., et al.
(författare)
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Organic data centers : A sustainable solution for computing facilities
- 2019
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Ingår i: Results in Engineering. - : Elsevier B.V.. - 2590-1230. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- In the present perspective article we provide an overview of on-going work in the literature and possible future development of organic data centers (ODC). These are defined as the combined operation of a data center and a greenhouse, and given their compatible thermal and operation requirements, ODCs have the potential to provide an excellent solution in terms of sustainability. In particular, we identify possible positive impacts of ODCs on at least 5 of the 17 United Nations (UN) Sustainable Development Goals (SDGs), including SDGs 2 and 13 on zero hunger and climate change, respectively.
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| 16. |
- Köpp, Wiebke, 1989-, et al.
(författare)
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Notes on Percolation Analysis of Sampled Scalar Fields
- 2019
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Percolation analysis is used to explore the connectivity of randomly connected infinite graphs. In the finite case, a closely related percolation function captures the relative volume of the largest connected component in a scalar field's-super level set. While prior work has shown that random scalar fields with little spatial correlation yield a sharp transition in this function, little is known about its behavior on real data. In this work, we explore how different characteristics of a scalar field – such as its histogram or degree of structure – influence the shape of the percolation function. We estimate the critical value and transition width of the percolation function, and propose a corresponding normalization scheme that relates these values to known results on infinite graphs. In our experiments, we find that percolation analysis can be used to analyze the degree of structure in Gaussian random fields. On a simulated turbulent duct flow data set we observe that the critical values are stable and consistent across time. Our normalization scheme indeed aid comparison between data sets and relation to infinite graphs.
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| 17. |
- Marin, O., et al.
(författare)
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Characterization of the secondary flow in hexagonal ducts
- 2016
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 28:12
-
Tidskriftsartikel (refereegranskat)abstract
- In this work we report the results of DNSs and LESs of the turbulent flow through hexagonal ducts at friction Reynolds numbers based on centerplane wall shear and duct half-height Re-tau,Re- c similar or equal to 180, 360, and 550. The evolution of the Fanning friction factor f with Re is in very good agreement with experimental measurements. A significant disagreement between the DNS and previous RANS simulations was found in the prediction of the in-plane velocity, and is explained through the inability of the RANS model to properly reproduce the secondary flow present in the hexagon. The kinetic energy of the secondary flow integrated over the cross-sectional area < K >(yz) decreases with Re in the hexagon, whereas it remains constant with Re in square ducts at comparable Reynolds numbers. Close connection between the values of Reynolds stress (uw) over bar on the horizontal wall close to the corner and the interaction of bursting events between the horizontal and inclined walls is found. This interaction leads to the formation of the secondary flow, and is less frequent in the hexagon as Re increases due to the 120 degrees aperture of its vertex, whereas in the square duct the 90 degrees corner leads to the same level of interaction with increasing Re. Analysis of turbulence statistics at the centerplane and the azimuthal variance of the mean flow and the fluctuations shows a close connection between hexagonal ducts and pipe flows, since the hexagon exhibits near-axisymmetric conditions up to a distance of around 0.15D(H) measured from its center. Spanwise distributions of wall-shear stress show that in square ducts the 90 degrees corner sets the location of a high-speed streak at a distance z(nu)(+) similar or equal to 50 from it, whereas in hexagons the 120 degrees aperture leads to a shorter distance of z(nu)(+) similar or equal to 38. At these locations the root mean square of the wall-shear stresses exhibits an inflection point, which further shows the connections between the near-wall structures and the large-scale motions in the outer flow. Published by AIP Publishing.
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| 18. |
- Monnier, Bruno, et al.
(författare)
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Turbulent Structure of a Simplified Urban Fluid Flow Studied Through Stereoscopic Particle Image Velocimetry
- 2018
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Ingår i: Boundary-layer Meteorology. - : SPRINGER. - 0006-8314 .- 1573-1472. ; 166:2, s. 239-268
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Tidskriftsartikel (refereegranskat)abstract
- Stereoscopic particle image velocimetry was used to provide a three-dimensional characterization of the flow around a simplified urban model defined by a 5 by 7 array of blocks, forming four parallel streets, perpendicular to the incoming wind direction corresponding to a zero angle of incidence. Channeling of the flow through the array under consideration was observed, and its effect increased as the incoming wind direction, or angle of incidence (AOI), was changed from to , , and . The flow between blocks can be divided into two regions: a region of low turbulence kinetic energy (TKE) levels close to the leeward side of the upstream block, and a high TKE area close to the downstream block. The centre of the arch vortex is located in the low TKE area, and two regions of large streamwise velocity fluctuation bound the vortex in the spanwise direction. Moreover, a region of large spanwise velocity fluctuation on the downstream block is found between the vortex legs. Our results indicate that the reorientation of the arch vortex at increasing AOI is produced by the displacement of the different TKE regions and their interaction with the shear layers on the sides and top of the upstream and downstream blocks, respectively. There is also a close connection between the turbulent structure between the blocks and the wind gusts. The correlations among gust components were also studied, and it was found that in the near-wall region of the street the correlations between the streamwise and spanwise gusts were dominant for all four AOI cases. At higher wall-normal positions in the array, the correlation decreased with increasing AOI, whereas the coefficient increased as AOI increased, and at all three correlations exhibited relatively high values of around 0.4.
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| 19. |
- Nagib, H. M., et al.
(författare)
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Vorticity fluxes : A tool for three-dimensional and secondary flows in turbulent shear flows
- 2019
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Ingår i: Journal of Fluids and Structures. - : Academic Press. - 0889-9746 .- 1095-8622. ; 89, s. 39-48
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Tidskriftsartikel (refereegranskat)abstract
- In this work we extend the vorticity-flux approach, proposed by Brown and Roshko (2012) for the analysis of turbulent shear layers and wakes, to the study of secondary flows of Prandtl's second kind. To this end, we assess direct numerical simulations (DNSs) of turbulent flow through sinusoidal channels (Vidal et al., 2018a) at bulk Reynolds numbers Re h = 2500 and 5000, and with various wall wave parameters, leading to a range of secondary flow intensities. We find that the fluctuating vorticity-flux difference (w'omega(y)') over bar (+) - (v'omega(z)') over bar (+) is closely connected to the in-plane cross-flow, in particular the large negative values present around the wall peak, which enhance the transport of near-wall momentum towards the channel core. The tilting of sweep events at the wall valley is also connected to the secondary flow magnitude, and is associated with positive values of the fluctuating vorticity-flux difference. Furthermore, conditionally averaged fields show that, unlike what is observed in channels with flat walls, the behavior in the vorticity-flux field at the peak is mostly due to Q1 and Q4 events, which essentially tilt momentum towards the peak.
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| 20. |
- Negi, Prabal Singh, et al.
(författare)
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Unsteady aerodynamic effects in pitching airfoils studied through large-eddy simulations
- 2017
-
Ingår i: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017. - : International Symposium on Turbulence and Shear Flow Phenomena, TSFP10.
-
Konferensbidrag (refereegranskat)abstract
- Wall-resolved large-eddy simulations (LES) are utilized to investigate the flow-physics of an airfoil undergoing pitch oscillations. A relaxation-term (RT) based filtering procedure is employed to add limited high order dissipation to account for the dissipation from the smallest scales which are not resolved. Validation of the procedure is presented for turbulent channel flows and for flow around a wing section. The procedure is then used for the simulation of small-amplitude pitching airfoil at Rec = 100;000 with a reduced frequency k = 0:5. The investigation of the unsteady phenomenon is done in the context of a natural laminar flow airfoil, the performance of which depends critically on the suction side transition characteristics. The dynamic range of the pitch cycle sees the appearance, destabilization and disappearance of a laminar separation bubble at the leading edge. An abrupt change is seen in the lift coefficient, which is linked to a rapid movement of the transition point over the suction side. Destabilization of the laminar separation bubble is the cause of these rapid transition movements which occur near the end of the pitch-up phase of the cycle.
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| 21. |
- Negi, Prabal Singh, et al.
(författare)
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Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil
- 2018
-
Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier. - 0142-727X .- 1879-2278. ; 71, s. 378-391
-
Tidskriftsartikel (refereegranskat)abstract
- High-fidelity wall-resolved large-eddy simulations (LES) are utilized to investigate the flow-physics of small-amplitude pitch oscillations of an airfoil at Rec=100,000. The investigation of the unsteady phenomenon is done in the context of natural laminar flow airfoils, which can display sensitive dependence of the aerodynamic forces on the angle of attack in certain “off-design” conditions. The dynamic range of the pitch oscillations is chosen to be in this sensitive region. Large variations of the transition point on the suction-side of the airfoil are observed throughout the pitch cycle resulting in a dynamically rich flow response. Changes in the stability characteristics of a leading-edge laminar separation bubble has a dominating influence on the boundary layer dynamics and causes an abrupt change in the transition location over the airfoil. The LES procedure is based on a relaxation-term which models the dissipation of the smallest unresolved scales. The validation of the procedure is provided for channel flows and for a stationary wing at Rec=400,000.
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| 22. |
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| 23. |
- Noorani, Azad, et al.
(författare)
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Aspect ratio effect on particle transport in turbulent duct flows
- 2016
-
Ingår i: Physics of fluids. - : AMER INST PHYSICS. - 1070-6631 .- 1089-7666. ; 28:11
-
Tidskriftsartikel (refereegranskat)abstract
- The dynamics of dilute micron-sized spherical inertial particles in turbulent duct flows is studied by means of direct numerical simulations of the carrier phase turbulence with one-way coupled Lagrangian particles. The geometries are a square and a rectangular duct with width-to-height aspect ratio AR of 3 operating at Re-tau,Re-c = 360 (based on the centerplane friction velocity and duct half-height). The present study is designed to determine the effect of turbulence-driven secondary motion on the particle dynamics. Our results show that a weak cross-flow secondary motion significantly changes the cross-sectional map of the particle concentration, mean velocity, and fluctuations. As the geometry of the duct is widened from AR = 1 to 3, the secondary vortex on the horizontal wall significantly expands in the spanwise direction, and although the kinetic energy of the secondary flow increases close to the corner, it decays towards the duct centreplane in the AR = 3 case so as the turbulent carrier phase approaches the behavior in spanwise-periodic channel flows, a fact that significantly affects the particle statistics. In the square duct the particle concentration in the viscous sublayer is maximum at the duct centreplane, whereas the maximum is found closer to the corner, at a distance of |z/h| approximate to 1.25 from the centreplane, in the AR = 3 case. Interestingly the centreplane concentration in the rectangular duct is around 3 times lower than that in the square duct. Moreover, a second peak in the accumulation distribution is found right at the corners for both ducts. At this location the concentration increases with particle inertia. The secondary motion changes also the cross-stream map of the particle velocities significantly in comparison to the fluid flow statistics. These directly affect the particle velocity fluctuations such that multiple peaks appear near the duct walls for the particle streamwise and wall-normal velocity fluctuations.
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| 24. |
- Otero, Evelyn, 1983-, et al.
(författare)
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Lossy Data Compression Effects on Wall-bounded Turbulence : Bounds on Data Reduction
- 2018
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Ingår i: Flow Turbulence and Combustion. - : Springer. - 1386-6184 .- 1573-1987. ; 101:2, s. 365-387
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Tidskriftsartikel (refereegranskat)abstract
- Postprocessing and storage of large data sets represent one of the main computational bottlenecks in computational fluid dynamics. We assume that the accuracy necessary for computation is higher than needed for postprocessing. Therefore, in the current work we assess thresholds for data reduction as required by the most common data analysis tools used in the study of fluid flow phenomena, specifically wall-bounded turbulence. These thresholds are imposed a priori by the user in L (2)-norm, and we assess a set of parameters to identify the minimum accuracy requirements. The method considered in the present work is the discrete Legendre transform (DLT), which we evaluate in the computation of turbulence statistics, spectral analysis and resilience for cases highly-sensitive to the initial conditions. Maximum acceptable compression ratios of the original data have been found to be around 97%, depending on the application purpose. The new method outperforms downsampling, as well as the previously explored data truncation method based on discrete Chebyshev transform (DCT).
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| 25. |
- Otero, Evelyn, 1983-, et al.
(författare)
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The effect of lossy data compression in computational fluid dynamics applications : Resilience and data postprocessing
- 2019
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Ingår i: Direct and Large-Eddy Simulation XI. - Cham : Springer. ; , s. 175-181
-
Bokkapitel (refereegranskat)abstract
- The field of computational fluid dynamics (CFD) is data intensive, particularly for high-fidelity simulations. Direct and large-eddy simulations (DNS and LES), which are framed in this high-fidelity regime, require to capture a wide range of flow scales, a fact that leads to a high number of degrees of freedom. Besides the computational bottleneck, brought by the size of the problem, a slightly overlooked issue is the manipulation of the data. High amounts of disk space and also the slow speed of I/O (input/output) impose limitations on large-scale simulations. Typically the computational requirements for proper resolution of the flow structures are far higher than those of post-processing. To mitigate such shortcomings we employ a lossy data compression procedure, and track the reduction that occurs for various levels of truncation of the data set.
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| 26. |
- Prus, C., et al.
(författare)
-
Impact simulation and optimisation of elastic fuel tanks reinforced with exoskeleton for aerospace applications
- 2017
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Ingår i: International Journal of Crashworthiness. - : Taylor & Francis. - 1358-8265 .- 1754-2111. ; 22:3, s. 271-293
-
Tidskriftsartikel (refereegranskat)abstract
- The main subject of the study is the impact simulation of an elastic fuel tank reinforced with a polymer exoskeleton. Thanks to its lightweight and failure resistance, this type of design shows potential to be used in aerospace applications. The simulation emulates a drop test from the height of 20 m on a rigid surface, in accordance with Military Handbook testing guidelines for fuel tanks. The focus is on providing an example of modelling and solving this type of problems. The computational methods are tested on a generic model of a rectangular prismatic tank with rounded edges. The walls of the tank are made of orthotropic fabric reinforced polymer. The simulation is performed for a 70% and a 100% water-filled tank. All calculations are performed using the Altair HyperWorks 13.0 software suite, in particular, the nonlinear RADIOSS solver and OptiStruct Solver and Optimiser. The fluid inside the tank is modelled using the SPH (Smoothed Particle Hydrodynamics) approach. The model serves as a basis for establishing a design optimisation procedure, aiming at reduction of mass of the tank components while ensuring structural integrity. The main insights of the current study are the successful modelling of the liquid and the air inside the tank by means of smoothed-particle hydrodynamics elements, and the structural optimisation methodology of a composite fuel tank.
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| 27. |
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| 28. |
- Rezaeiravesh, Saleh, et al.
(författare)
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Assessment of uncertainties in hot-wire anemometry and oil-film interferometry measurements for wall-bounded turbulent flows
- 2018
-
Ingår i: European journal of mechanics. B, Fluids. - : Elsevier. - 0997-7546 .- 1873-7390. ; 72, s. 57-73
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, the sources of uncertainty of hot-wire anemometry (HWA) and oil-film interferometry (OFI) measurements are assessed. Both statistical and classical methods are used for the forward and inverse problems, so that the contributions to the overall uncertainty of the measured quantities can be evaluated. The correlations between the parameters are taken into account through the Bayesian inference with error-in-variable (EiV) model. In the forward problem, very small differences were found when using Monte Carlo (MC), Polynomial Chaos Expansion (PCE) and linear perturbation methods. In flow velocity measurements with HWA, the results indicate that the estimated uncertainty is lower when the correlations among parameters are considered, than when they are not taken into account. Moreover, global sensitivity analyses with Sobol indices showed that the HWA measurements are most sensitive to the wire voltage, and in the case of OFI the most sensitive factor is the calculation of fringe velocity. The relative errors in wall-shear stress, friction velocity and viscous length are 0.44%, 0.23% and0.22%, respectively. Note that these values are lower than the ones reported in other wall-bounded turbulence studies. Note that in most studies of wall-bounded turbulence the correlations among parameters are not considered, and the uncertainties from the various parameters are directly added when determining the overall uncertainty of the measured quantity. In the present analysis we account for these correlations, which may lead to a lower overall uncertainty estimate due to error cancellation Furthermore, our results also indicate that the crucial aspect when obtaining accurate inner-scaled velocity measurements is the wind-tunnel flow quality, which is more critical than the accuracy in wall-shear stress measurements.
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| 29. |
- Ryzhenkov, V., et al.
(författare)
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Simulation of heat and mass transfer in turbulent channel flow using the spectral-element method : Effect of spatial resolution
- 2016
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Ingår i: Journal of Physics, Conference Series. - : Institute of Physics (IOP). - 1742-6588 .- 1742-6596. ; 754:6
-
Tidskriftsartikel (refereegranskat)abstract
- We use the open-source code nek5000 to assess the accuracy of high-order spectral element large-eddy simulations (LES) of a turbulent channel flow depending on the spatial resolution compared to the direct numerical simulation (DNS). The Reynolds number Re = 6800 is considered based on the bulk velocity and half-width of the channel. The filtered governing equations are closed with the dynamic Smagorinsky model for subgrid stresses and heat flux. The results show very good agreement between LES and DNS for time-averaged velocity and temperature profiles and their fluctuations. Even the coarse LES grid which contains around 30 times less points than the DNS one provided predictions of the friction velocity within 2.0% accuracy interval.
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| 30. |
- Ryzhenkov, V., et al.
(författare)
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Spectral-element simulations of variable-density turbulent flow in a plane channel
- 2017
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Ingår i: EPJ Web of Conferences. - : EDP Sciences. - 2100-014X.
-
Konferensbidrag (refereegranskat)abstract
- We perform Large-eddy simulations (LES) of the turbulent flow in a channel with isothermal heated walls with the temperature ratio equal to 2. The variable properties of the fluid are accounted for by using the low Mach number approximation. The Reynolds number based on the bulk velocity, half-width of the channel, density and dynamic viscosity near the cold wall is 6800. We study the effect of spatial resolution on the accuracy of Large-eddy simulations with dynamic Smagorinsky model. The very good agreement of LES results is shown even for the coarse meshes which is attributed to the high accuracy of the spectral method.
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| 31. |
- Samanta, Arghya, et al.
(författare)
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Direct numerical simulations of turbulent flow through porous channels and ducts
- 2015
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Ingår i: Proceedings - 15th European Turbulence Conference, ETC 2015. - : TU Delft.
-
Konferensbidrag (refereegranskat)abstract
- Direct numerical simulations of the fully developed turbulent flow through a porous channel and duct are performed based on the spectral element code Nek5000. The volume-averaged Navier-Stokes (VANS) equations are implemented in order to describe the flow in the composite medium. The numerical simulations of the VANS equations are carried out at a constant value of the bulk Reynolds number when the porosity, or equivalently, the permeability of the medium varies successively. The mean and turbulent energy budgets are computed and the effect of porosity on the secondary flow in a duct is examined.
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| 32. |
- Samanta, Arghya, et al.
(författare)
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Enhanced secondary motion of the turbulent flow through a porous square duct
- 2015
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 784, s. 681-693
-
Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulations of the fully developed turbulent flow through a porous square duct are performed to study the effect of the permeable wall on the secondary cross-stream flow. The volume-averaged Navier-Stokes equations are used to describe the flow in the porous phase, a packed bed with porosity epsilon(c) = 0.95. The porous square duct is computed at Re-b similar or equal to 5000 and compared with the numerical simulations of a turbulent duct with four solid walls. The two boundary layers on the top wall and porous interface merge close to the centre of the duct, as opposed to the channel, because the sidewall boundary layers inhibit the growth of the shear layer over the porous interface. The most relevant feature in the porous duct is the enhanced magnitude of the secondary flow, which exceeds that of a regular duct by a factor of four. This is related to the increased vertical velocity, and the different interaction between the ejections from the sidewalls and the porous medium. We also report a significant decrease in the streamwise turbulence intensity over the porous wall of the duct (which is also observed in a porous channel), and the appearance of short spanwise rollers in the buffer layer, replacing the streaky structures of wall-bounded turbulence. These spanwise rollers most probably result from a Kelvin-Helmholtz type of instability, and their width is limited by the presence of the sidewalls.
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| 33. |
- Sanmiguel Vila, Carlos, et al.
(författare)
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Adverse-Pressure-Gradient Effects on Turbulent Boundary Layers : Statistics and Flow-Field Organization
- 2017
-
Ingår i: Flow Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1386-6184 .- 1573-1987. ; 99:3-4, s. 589-612
-
Tidskriftsartikel (refereegranskat)abstract
- This manuscripts presents a study on adverse-pressure-gradient turbulent boundary layers under different Reynolds-number and pressure-gradient conditions. In this work we performed Particle Image Velocimetry (PIV) measurements supplemented with Large-Eddy Simulations in order to have a dataset covering a range of displacement-thickness-based Reynolds-number 2300 34000 and values of the Clauser pressure-gradient parameter beta up to 2.4. The spatial resolution limits of PIV for the estimation of turbulence statistics have been overcome via ensemble-based approaches. A comparison between ensemble-correlation and ensemble Particle Tracking Velocimetry was carried out to assess the uncertainty of the two methods. The effects of beta, R e and of the pressure-gradient history on turbulence statistics were assessed. A modal analysis via Proper Orthogonal Decomposition was carried out on the flow fields and showed that about 20% of the energy contribution corresponds to the first mode, while 40% of the turbulent kinetic energy corresponds to the first four modes with no appreciable dependence on beta and R e within the investigated range. The topology of the spatial modes shows a dependence on the Reynolds number and on the pressure-gradient strength, in line with the results obtained from the analysis of the turbulence statistics. The contribution of the modes to the Reynolds stresses and the turbulence production was assessed using a truncated low-order reconstruction with progressively larger number of modes. It is shown that the outer peaks in the Reynolds-stress profiles are mostly due to large-scale structures in the outer part of the boundary layer.
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| 34. |
- Sanmiguel Vila, C., et al.
(författare)
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Identifying well-behaved turbulent boundary layers
- 2017
-
Ingår i: Progress in Turbulence VII. - Cham : Springer Science+Business Media B.V.. - 9783319579337 ; , s. 67-72
-
Konferensbidrag (refereegranskat)abstract
- This paper presents a study focused on the development of zero-pressure-gradient turbulent boundary layers (ZPG TBL) towards well-behaved conditions in the low Reynolds-number range. A new method to assess the length required for the ZPG TBL to exhibit well-behaved conditions is proposed. The proposed method is based on the diagnostic-plot concept (Alfredsson et al., Phys. Fluids, 23:041702, 2011), which only requires mean and turbulence intensity measurements in the outer region of the boundary layer. In contrast to the existing methods which rely on empirical skin-friction curves, shape-factor or wake-parameter, the quantities required by this method are generally much easier to measure. To test the method, the evolution of six different tripping configurations, including weak, late and strong overtripping, are studied in a wind-tunnel experiment to assess the convergence of ZPG TBLs towards well-behaved conditions in the momentum-thickness based Reynolds-number range 500 < Reθ < 4000.
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| 35. |
- Sanmiguel Vila, C., et al.
(författare)
-
Large-scale energy in turbulent boundary layers : Reynolds-number and pressure-gradient effects
- 2019
-
Ingår i: Springer Proceedings in Physics. - Cham : Springer Nature. ; , s. 69-74
-
Konferensbidrag (refereegranskat)abstract
- Adverse-pressure-gradient (APG) turbulent boundary layers (TBLs) are studied using hot-wire measurements which cover a Clauser pressure-gradient-parameter range up to β ≈ 2.4. Constant and non-constant β distributions with the same upstream history are studied. The pre-multiplied power-spectral density is employed to study the differences in the large-scale energy content throughout the boundary layer. Two different large-scale phenomena are identified, the first one due to the pressure gradient and the second one due to the Reynolds number; the latter is also present in high-Re ZPG TBLs. A decomposition of the streamwise velocity fluctuations using a temporal filter shows that the small-scale velocity fluctuations do not scale in APG TBL flows since the effect of the large-scale features extends up to the near-wall region.
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| 36. |
- Sanmiguel Vila, C., et al.
(författare)
-
On the identification of well-behaved turbulent boundary layers
- 2017
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 822, s. 109-138
-
Tidskriftsartikel (refereegranskat)abstract
- This paper introduces a new method based on the diagnostic plot (Alfredsson et al., Phys. Fluids, vol. 23, 2011, 041702) to assess the convergence towards a well-behaved zero-pressure-gradient (ZPG) turbulent boundary layer (TBL). The most popular and well-understood methods to assess the convergence towards a well-behaved state rely on empirical skin-friction curves (requiring accurate skin-friction measurements), shape-factor curves (requiring full velocity profile measurements with an accurate wall position determination) or wake-parameter curves (requiring both of the previous quantities). On the other hand, the proposed diagnostic-plot method only needs measurements of mean and fluctuating velocities in the outer region of the boundary layer at arbitrary wall-normal positions. To test the method, six tripping configurations, including optimal set-ups as well as both under- and overtripped cases, are used to quantify the convergence of ZPG TBLs towards well-behaved conditions in the Reynolds-number range covered by recent high-fidelity direct numerical simulation data up to a Reynolds number based on the momentum thickness and free-stream velocity of approximately 4000 (corresponding to 2.5 m from the leading edge) in a wind-tunnel experiment. Additionally, recent high-Reynolds-number data sets have been employed to validate the method. The results show that weak tripping configurations lead to deviations in the mean flow and the velocity fluctuations within the logarithmic region with respect to optimally tripped boundary layers. On the other hand, a strong trip leads to a more energized outer region, manifested in the emergence of an outer peak in the velocity-fluctuation profile and in a more prominent wake region. While established criteria based on skin-friction and shape-factor correlations yield generally equivalent results with the diagnostic-plot method in terms of convergence towards a well-behaved state, the proposed method has the advantage of being a practical surrogate that is a more efficient tool when designing the set-up for TBL experiments, since it diagnoses the state of the boundary layer without the need to perform extensive velocity profile measurements.
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| 37. |
- Sasaki, Kenzo, et al.
(författare)
-
Transfer functions for flow predictions in wall-bounded turbulence
- 2019
-
Ingår i: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 864, s. 708-745
-
Tidskriftsartikel (refereegranskat)abstract
- Three methods are evaluated to estimate the streamwise velocity fluctuations of a zero-pressure-gradient turbulent boundary layer of momentum-thickness-based Reynolds number up to using as input velocity fluctuations at different wall-normal positions. A system identification approach is considered where large-eddy simulation data are used to build single and multiple-input linear and nonlinear transfer functions. Such transfer functions are then treated as convolution kernels and may be used as models for the prediction of the fluctuations. Good agreement between predicted and reference data is observed when the streamwise velocity in the near-wall region is estimated from fluctuations in the outer region. Both the unsteady behaviour of the fluctuations and the spectral content of the data are properly predicted. It is shown that approximately 45 % of the energy in the near-wall peak is linearly correlated with the outer-layer structures, for the reference case. These identified transfer functions allow insight into the causality between the different wall-normal locations in a turbulent boundary layer along with an estimation of the tilting angle of the large-scale structures. Differences in accuracy of the methods (single- and multiple-input linear and nonlinear) are assessed by evaluating the coherence of the structures between wall-normally separated positions. It is shown that the large-scale fluctuations are coherent between the outer and inner layers, by means of an interactions which strengthens with increasing Reynolds number, whereas the finer-scale fluctuations are only coherent within the near-wall region. This enables the possibility of considering the wall-shear stress as an input measurement, which would more easily allow the implementation of these methods in experimental applications. A parametric study was also performed by evaluating the effect of the Reynolds number, wall-normal positions and input quantities considered in the model. Since the methods vary in terms of their complexity for implementation, computational expense and accuracy, the technique of choice will depend on the application under consideration. We also assessed the possibility of designing and testing the models at different Reynolds numbers, where it is shown that the prediction of the near-wall peak from wall-shear-stress measurements is practically unaffected even for a one order of magnitude change in the corresponding Reynolds number of the design and test, indicating that the interaction between the near-wall peak fluctuations and the wall is approximately Reynolds-number independent. Furthermore, given the performance of such methods in the prediction of flow features in turbulent boundary layers, they have a good potential for implementation in experiments and realistic flow control applications, where the prediction of the near-wall peak led to correlations above 0.80 when wall-shear stress was used in a multiple-input or nonlinear scheme. Errors of the order of 20 % were also observed in the determination of the near-wall spectral peak, depending on the employed method.
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| 38. |
- Schenk, F., et al.
(författare)
-
Enhanced large-scale atmospheric flow interaction with ice sheets at high model resolution
- 2019
-
Ingår i: Results in Engineering. - : Elsevier. - 2590-1230. ; 3
-
Tidskriftsartikel (refereegranskat)abstract
- The development in supercomputing power allows running full-complexity Earth System Models (ESM) at increasingly higher spatial resolutions on a global scale. We show here a recent example where increased model resolution leads to a fundamentally different large-scale fluid dynamical adjustment of the mean wind pattern to the presence of an ice sheet over Europe compared to a coarse resolution simulation. While the higher resolution allows for a more realistic representation of atmospheric flow interaction with complex topographic features, the interpretation and prediction of the model results with a stronger bottom-up mechanical and thermal forcing on the atmosphere becomes increasingly difficult to be studied within a fully coupled model. We emphasize that interdisciplinary approaches should be pursued where the experience from engineering approaches of studying flow around objects and the influence of boundary-layer processes can help to disentangle the complexity within ESM. Ultimately, such engineering approaches will add a more fundamental theoretical understanding and prediction of expected flow interactions and will help to design full-complexity atmospheric model experiments accordingly.
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| 39. |
- Srinivasan, P. A., et al.
(författare)
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Predictions of turbulent shear flows using deep neural networks
- 2019
-
Ingår i: Physical Review Fluids. - : AMER PHYSICAL SOC. - 2469-990X. ; 4:5
-
Tidskriftsartikel (refereegranskat)abstract
- In the present work, we assess the capabilities of neural networks to predict temporally evolving turbulent flows. In particular, we use the nine-equation shear flow model by Moehlis et al. [New J. Phys. 6, 56 (2004)] to generate training data for two types of neural networks: the multilayer perceptron (MLP) and the long short-term memory (LSTM) networks. We tested a number of neural network architectures by varying the number of layers, number of units per layer, dimension of the input, and weight initialization and activation functions in order to obtain the best configurations for flow prediction. Because of its ability to exploit the sequential nature of the data, the LSTM network outperformed the MLP. The LSTM led to excellent predictions of turbulence statistics (with relative errors of 0.45% and 2.49% in mean and fluctuating quantities, respectively) and of the dynamical behavior of the system (characterized by Poincare maps and Lyapunov exponents). This is an exploratory study where we consider a low-order representation of near-wall turbulence. Based on the present results, the proposed machine-learning framework may underpin future applications aimed at developing accurate and efficient data-driven subgrid-scale models for large-eddy simulations of more complex wall-bounded turbulent flows, including channels and developing boundary layers.
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| 40. |
- Straub, Steffen, et al.
(författare)
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The influence of thermal boundary conditions on turbulent forced convection pipe flow at two Prandtl numbers
- 2019
-
Ingår i: International Journal of Heat and Mass Transfer. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0017-9310 .- 1879-2189. ; 144
-
Tidskriftsartikel (refereegranskat)abstract
- Different types of thermal boundary conditions are conceivable in numerical simulations of convective heat transfer problems. Isoflux, isothermal and a mixed-type boundary condition are compared by means of direct numerical simulations (for the lowest Reynolds number) and well-resolved large-eddy simulations of a turbulent forced convection pipe flow over a range of bulk Reynolds numbers from Re-b = 5300 to Re-b = 37700, at two Prandtl numbers, i.e. Pr = 0.71 and Pr = 0.025. It is found that, while for Pr = 0.71 the Nusselt number is hardly affected by the type of thermal boundary condition, for Pr = 0.025 the isothermal boundary condition yields approximate to 20% lower Nusselt numbers compared to isoflux and mixedtype over the whole range of Reynolds numbers. A decomposition of the Nusselt number is derived. In particular, we decompose it into four contributions: laminar, radial and streamwise turbulent heat flux as well as a contribution due to the turbulent velocity field. For Pr = 0.71 the contribution due to the radial turbulent heat flux is dominant, whereas for Pr = 0.025 the contribution due to the turbulent velocity field is dominant. Only at a moderately high Reynolds number, such as Re-b = 37700, both turbulent contributions are of similar magnitude. A comparison of first- and second-order thermal statistics between the different types of thermal boundary conditions shows that the statistics are not only influenced in the near-wall region but also in the core region of the flow. Power spectral densities illustrate large thermal structures in low-Prandtl-number fluids as well as thermal structures located right at the wall, only present for the isoflux boundary condition. A database including the first- and second-order statistics together with individual contributions to the budget equations of the temperature variance and turbulent heat fluxes is hosted in the open access repository KITopen (DOI : https: //doi.org/10.5445/IR/1000096346).
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| 41. |
- Straub, Steffen, et al.
(författare)
-
Turbulent Duct Flow Controlled with Spanwise Wall Oscillations
- 2017
-
Ingår i: Flow Turbulence and Combustion. - : Springer. - 1386-6184 .- 1573-1987. ; 99:3-4, s. 787-806
-
Tidskriftsartikel (refereegranskat)abstract
- The spanwise oscillation of channel walls is known to substantially reduce the skin-friction drag in turbulent channel flows. In order to understand the limitations of this flow control approach when applied in ducts, direct numerical simulations of controlled turbulent duct flows with an aspect ratio of A R = 3 are performed. In contrast to channel flows, the spanwise extension of the duct is limited. Therefore, the spanwise wall oscillation either directly interacts with the duct side walls or its spatial extent is limited to a certain region of the duct. The present results show that this spanwise limitation of the oscillating region strongly diminishes the drag reduction potential of the control technique. We propose a simple model that allows estimating the achievable drag reduction rates in duct flows as a function of the width of the duct and the spanwise extent of the controlled region.
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| 42. |
- Tanarro, Álvaro, et al.
(författare)
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Using adaptive mesh refinement to simulate turbulent wings at high Reynolds numbers
- 2019
-
Ingår i: 11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019.
-
Konferensbidrag (refereegranskat)abstract
- The implementation of adaptive mesh refinement (AMR) in Nek5000 is used for the first time on the simulation of the flow over wings. This is done by simulating the flow over a NACA4412 profile with 5 degrees angle of attack at chord-based Reynolds number 200,000. The mesh is progressively refined by means of AMR which allows for high resolution near the wall whereas significantly larger elements are used in the far-field. The resultant mesh shows higher resolution than previous conformal meshes, and it allows for larger computational domains,which avoid the use of RANS to determine the boundary condition, all of this with, approximately, 3 times lower total number of grid points. The results ofthe turbulence statistics show a good agreement with the ones obtained with the conformal mesh. Finally, using AMR on wings leads to simulations at higher Reynolds numbers (i.e. Rec = 850, 000) in order to analyse the effect of adverse pressure gradients at high Reynolds numbers.
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| 43. |
- Vidal, A., et al.
(författare)
-
Impact of corner geometry on the secondary flow in turbulent ducts
- 2017
-
Ingår i: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017. - : International Symposium on Turbulence and Shear Flow Phenomena, TSFP10. - 9780000000002
-
Konferensbidrag (refereegranskat)abstract
- In the present study we perform direct numerical simulations (DNSs) of fully-developed turbulent square ducts with round corners at Reτ,c ∼ 180 and 360, and rectangular ducts of width-toheight ratios of 3 and 5 with rounded side walls at Reτ,c ∼ 180. The friction Reynolds number Reτ,c is based on the centerplane friction velocity and the half-height of the duct. The results are compared with the corresponding duct cases with 90° corners. We focus on the influence of the rounding on the mean cross-stream secondary flow and on further characterizing the mechanisms that produce it. Unexpectedly, the rounded ducts exhibit higher cross-flow rates and their secondary vortices relocate near the transition point between the straight and curved walls. This behavior is associated to the statistically preferential arrangement of sweeping events entering through the curved wall, which trigger an ejection on the adjacent straight wall. We have yet to find effective modifications to the corners or transverse ends of a rectangular duct that would render better rigorous modeling of two-dimensional channel flows.
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| 44. |
- Vidal, A., et al.
(författare)
-
Influence of corner geometry on the secondary flow in turbulent square ducts
- 2017
-
Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier. - 0142-727X .- 1879-2278. ; 67, s. 69-78
-
Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulations of fully-developed turbulent flow through a straight square duct with increasing corner rounding radius r were performed to study the influence of corner geometry on the secondary flow. Unexpectedly, the increased rounding of the corners from r=0 to 0.75 does not lead to a monotonic trend towards the pipe case of r=1. Instead, the secondary vortices relocate close to the region of wall-curvature change. This behavior is connected to the inhomogeneous interaction between near-wall bursting events, which are further characterized in this work with the definition of their local preferential direction. We compare our results with those obtained for the flow through a square duct (which corresponds to r=0) and through a round pipe (r=1), focusing on the influence of r on the wall-shear stress distribution and the turbulence statistics along the centerplane and the corner bisector. The former shows that high-speed streaks are preferentially located near the transition between straight and curved surfaces. The Reynolds numbers based on the centerplane friction velocity and duct half-height are Reτ, c ≃ 180 and 350 for the cases under study.
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| 45. |
- Vidal, A., et al.
(författare)
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Influence of corner geometry on the secondary flow in turbulent square ducts (Reprint from Int J Heat and Fluid Flow 67A, pp.69-78, 2017)
- 2017
-
Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier BV. - 0142-727X .- 1879-2278. ; 67, s. 94-103
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Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulations of fully-developed turbulent flow through a straight square duct with increasing corner rounding radius r were performed to study the influence of corner geometry on the secondary flow. Unexpectedly, the increased rounding of the corners from r = 0 to 0.75 does not lead to a monotonic trend towards the pipe case of r = 1. Instead, the secondary vortices relocate close to the region of wall-curvature change. This behavior is connected to the inhomogeneous interaction between near-wall bursting events, which are further characterized in this work with the definition of their local preferential direction. We compare our results with those obtained for the flow through a square duct (which corresponds to r = 0) and through a round pipe (r = 1), focusing on the influence of r on the wall-shear stress distribution and the turbulence statistics along the centerplane and the corner bisector. The former shows that high-speed streaks are preferentially located near the transition between straight and curved surfaces. The Reynolds numbers based on the centerplane friction velocity and duct half-height are Re-r,Re-
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| 46. |
- Vidal, A., et al.
(författare)
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Secondary flow in spanwise-periodic in-phase sinusoidal channels
- 2018
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 851, s. 288-316
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Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulations (DNSs) are performed to analyse the secondary flow of Prandtl's second kind in fully developed spanwise-periodic channels with in-plane sinusoidal walls. The secondary flow is characterized for different combinations of wave parameters defining the wall geometry at Re-h = 2500 and 5000, where h is the half-height of the channel. The total cross-flow rate in the channel Q(yz) is defined along with a theoretical model to predict its behaviour. Interaction between the secondary flows from opposite walls is observed if lambda similar or equal to h similar or equal to A, where A and lambda are the amplitude and wavelength of the sinusoidal function defining the wall geometry. As the outer-scaled wavelength (lambda/h) is reduced, the secondary vortices become smaller and faster, increasing the total cross-flow rate per wall. However, if the inner-scaled wavelength (lambda(+)) is below 130 viscous units, the cross-flow decays for smaller wavelengths. By analysing cases in which the wavelength of the wall is much smaller than the half-height of the channel lambda << h, we show that the cross-flow distribution depends almost entirely on the separation between the scales of the instantaneous vortices, where the upper and lower bounds are determined by lambda/h and lambda(+), respectively. Therefore, the distribution of the secondary flow relative to the size of the wave at a given Re-h can be replicated at higher Re-h by decreasing lambda/h and keeping lambda(+) constant. The mechanisms that contribute to the mean cross-flow are analysed in terms of the Reynolds stresses and using quadrant analysis to evaluate the probability density function of the bursting events. These events are further classified with respect to the sign of their instantaneous spanwise velocities. Sweeping events and ejections are preferentially located in the valleys and peaks of the wall, respectively. The sweeps direct the instantaneous cross-flow from the core of the channel towards the wall, turning in the wall-tangent direction towards the peaks. The ejections drive the instantaneous cross-flow from the near-wall region towards the core. This preferential behaviour is identified as one of the main contributors to the secondary flow.
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| 47. |
- Vidal, A., et al.
(författare)
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Turbulent rectangular ducts with minimum secondary flow
- 2018
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Ingår i: International Journal of Heat and Fluid Flow. - : ELSEVIER SCIENCE INC. - 0142-727X .- 1879-2278. ; 72, s. 317-328
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Tidskriftsartikel (refereegranskat)abstract
- In the present study we perform direct numerical simulations (DNSs) of fully-developed turbulent rectangular ducts with semi-cylindrical side-walls at Re-t,Re- c similar or equal to 180 with width-to-height ratios of 3 and 5. The friction Reynolds number Re-tau,Re- (c) is based on the centerplane friction velocity and the half-height of the duct. The results are compared with the corresponding duct cases with straight side-walls (Vinuesa et al., 2014), and also with spanwise-periodic channel and pipe flows. We focus on the influence of the semi-cylindrical side-walls on the mean cross-stream secondary flow and on further characterizing the mechanisms that produce it. The role of the secondary and primary Reynolds-shear stresses in the production of the secondary flow is analyzed by means of quadrant analysis and conditional averaging. Unexpectedly, the ducts with semi-cylindrical side-walls exhibit higher cross-flow rates and their secondary vortices relocate near the transition point between the straight and curved walls. This behavior is associated to the statistically preferential arrangement of sweeping events entering through the curved wall and ejections arising from the adjacent straight wall. Therefore, the configuration with minimum secondary flow corresponds to the duct with straight side-walls and sharp corners. Consequences on experimental facilities and comparisons between experiments and various numerical and theoretical models are discussed revealing the uniqueness of pipe flow.
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| 48. |
- Vidal, A., et al.
(författare)
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Vorticity fluxes and secondary flow : Relevance for turbulence modeling
- 2018
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Ingår i: Physical Review Fluids. - : American Physical Society. - 2469-990X. ; 3:7
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Tidskriftsartikel (refereegranskat)abstract
- Vorticity fluxes are analyzed in fully developed turbulent flow through rectangular ducts with a width-to-height ratio of 3, and both straight and semicylindrical side walls, at a centerplane friction Reynolds number Re-tau,(c) similar or equal to 180. The transport of secondary Reynolds stresses by the secondary flow of Prandtl's second kind is analyzed from a vorticity-flux perspective. This analysis reveals that the in-plane transport of viscous stresses locally counteracts the inhomogeneous distribution of the turbulent shear-stress gradient in the spanwise direction. A relationship is established between the mean and fluctuating transport terms that can be useful to improve turbulence models and their ability to accurately predict the secondary flow. Finally, quadrant analysis is used to evaluate the contribution from the different types of bursting events to the fluctuating transport terms.
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| 49. |
- Vinuesa, Ricardo, et al.
(författare)
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Alternative interpretation of the Superpipe data and motivation for CICLoPE : The effect of a decreasing viscous length scale
- 2016
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier. - 0997-7546 .- 1873-7390. ; 58, s. 109-116
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Tidskriftsartikel (refereegranskat)abstract
- Pressurization and cryogenic conditions have been used in some experiments to change the kinematic viscosity v of the flowing gas by many orders of magnitude in order to achieve high Reynolds number conditions in facilities of limited size. This leads to a substantial reduction of the viscous length scale l* = v/u(tau), as in the so-called Princeton "Superpipe" experiments, We demonstrate that the limited dimensions of the facilities and probes can lead to inaccuracies in the near-wall measurements for increasing Reynolds number. Specifically, a lack of accurate wall-normal probe positioning is simulated using three different datasets of wall-bounded turbulent flows. Relatively large errors in the overlap region parameters are observed for position errors of small physical magnitude that become greatly amplified in wall units as l* is reduced. This offers an alternative interpretation to some of the key findings reported by the Superpipe team, such as the increasing lower limit of the logarithmic region y(log,min)(+), the existence of a power law region between the wall and the logarithmic layer, and the "mixing transition" phenomenon in wall bounded turbulence.
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| 50. |
- Vinuesa, Ricardo, et al.
(författare)
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Characterisation of backflow events over a wing section
- 2017
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Ingår i: Journal of Turbulence. - : Taylor & Francis. - 1468-5248. ; 18:2, s. 170-185
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Tidskriftsartikel (refereegranskat)abstract
- Rare backflow (negative wall-shear stress) events have recently been found and quantified in the near-wall region of canonical wall-bounded turbulent flows. Although their existence and correlation with large-scale events have been established beyond numerical and measurement technique uncertainties, their occurrence at numerically high Reynolds numbers is still rare (less than 1 per thousand and 1 per million at the wall and beyond the viscous sublayer, respectively). To better quantify these rare events, the turbulent boundary layer developing over the suction side of a wing section, experiencing an increasing adverse pressure gradient (APG) without separation along its chord c, is considered in the present work. We find that the backflow level of 0.06% documented in turbulent channels and zero-pressure-gradient (ZPG) turbulent boundary layers is already exceeded on the suction side for x/c > 0.3, at friction Reynolds numbers three times lower, while close to the trailing edge the backflow level reaches 30%. Conditional analysis of extreme events indicates that for increasing Clauser pressure-gradient parameters (reaching β ≃ 35), the flow reaches a state in which the extreme events are more likely aligned with or against the freestream, and that the otherwise strong spanwise component of the wall-shear stress reduces towards the vicinity of the trailing edge. Backflow events subjected to moderate up to strong APG conditions (0.6 < β < 4.1) exhibit an average width of Δz+ ≃ 20, and an average lifetime of Δt+ ≃ 2. This directly connects with the findings by Lenaers et al., and implies that there is a connection between high-Re ZPG and strong APG conditions.
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